As a forming method of microlenses on a substrate, various methods are studied.
As a first method, there is a well-known method (melt method) including: forming a photosensitive thermoplastic resin layer for lenses on a color filter for light receivers of an image sensor; exposing the resin layer to light using a photomask having a predetermined pattern and developing the resin layer to form a columnar resist pattern layer at a position corresponding to each light receiver; and subjecting the resist pattern layer to a heating treatment at a softening point of the thermoplastic resin or more so as to cause the resin to flow, so that the resin droops at edges of the pattern layer to form convex lenses.
As a second method, there is a known method (etch back method) including: forming a photosensitive resist film on a planarized resin layer for lenses on an image sensor; exposing the resist film to light using a photomask having a predetermined pattern and developing the resist film to form a columnar resist pattern layer at a position corresponding to each light receiver; subjecting the resist pattern layer to a heating treatment at a softening point of the resist or more so as to cause the resin to flow, so that the resin droops at edges of the pattern layer to form convex-lens shapes; and etching back the resin layer for lenses using the convex-lens shapes as an etching mask to remove the resist film and simultaneously to form lenses in the resin layer for lenses.
As a third method, there is a known method (gray scale method) including: forming a photosensitive curable resin layer for lenses on a color filter for light receivers; and exposing the resin layer to light through a gray scale mask for controlling the distribution of a transmitted light volume during the exposure depending on the distribution state of fine patterns incapable of being resolved at an exposure wavelength and developing the resin layer to directly form a lens pattern layer at a position corresponding to each light receiver.
When the first method of the related art is used, the pattern layer of the photosensitive thermoplastic resin for lenses is heated to flow to provide convex-lens shapes. Therefore, during the heating-to-flow process, a fusion is caused between the lenses adjacent to each other, which becomes a cause of the lowering of the yield. If the distance between the lenses is set large in order to prevent this, the yield is improved. However, a problem occurs that the performance of an image sensor degrades, because a gap is caused between the lenses and this leads to the lowering of the aperture ratio.
The second method also has a heating-to-flow process, so it has risks similar to the first method. Furthermore, the etch back process is essential to the second method, so that facilities such as a reactive ion etching apparatus are necessary and the process becomes lengthened. Accordingly, a defect during the etch back newly occurs, which affects the yield of the lens formation.
The third method is advantageous for shortening the process and making the aperture ratio high. However, following the reduction of the pixel size, it is difficult to exactly transfer a photomask pattern to a resist with the third method.
As a fourth method, there is a known method (melt method) including: transferring a resist pattern in a plaid pattern to a resin layer on a substrate for preventing a fusion between the lenses adjacent to each other; causing the resin layer to flow by heating and temporarily curing the resin layer; and forming a new resist pattern layer in a part having no pattern, causing the resist pattern layer to flow by heating, and curing the resist pattern layer. There is reported that microlenses having small gaps between the lenses can be formed by this method. However, even by this method, the gap between the lenses adjacent to each other cannot be eliminated. Furthermore, there is such a problem that a microlens array shape with an overlap between the lenses and a high aperture ratio, which is regarded as ideal for the collection efficiency, cannot be obtained.
Meanwhile, there is a method for coating the surfaces of microlenses with an overcoating layer (see Patent Document 1 and Patent Document 2). The method includes: forming the microlenses on a substrate; applying thereon an overcoating material to form the overcoating layer using lithography; and reflowing.
In addition, there is a method (see Patent Document 3) for forming a structure in which the surface of a microlens is coated with an overcoating layer. The method includes: coating the surfaces of microlenses with an overcoating material; exposing the overcoating material to light using the gray scale mask; and developing the overcoating material.